This invention relates to envelope controls used for electronic musical instruments such as music synthesizers for varying the musical sound envelope in the order of attack, decay, sustain and release status sections from the instant of depressing a key till an instant after the key is released.
The music synthesizer is usually provided with an envelope generator for converting the musical sound envelope into a desired form. The envelope control signal generated from such an envelope generator is used to control a voltage controlled oscillator (VCO), a voltage controlled filter (VCF) or a voltage controlled amplifier (VCA). Hitherto, an envelope generator which is constructed as an analog circuit has been constituted by a commonly termed CR charging/discharging circuit including a capacitor and a resistor. The capacitance of the capacitor and the resistance of the resistor are subject to changes in long use, and the changes of these circuit parameters result in changes of the characteristics of the envelope that is generated. In addition, since the CR charging/discharging circuit is constructed by using discrete capacitive and resistive elements, the envelope generator has a large size. Especially, in the case of a composite musical sound synthesizer with which various envelopes can be provided for a single musical tone with the operation of a select switch, envelope generators corresponding in number to the number of envelopes to be provided are required, thus leading to a very large size construction.